The field of the invention is the preparation of methyl tertiary butyl ether (MTBE) by the catalytic addition of methanol to isobutene. The state of the art of this preparation may be ascertained by reference to U.S. Pat. Nos. 2,480,940; 2,922,822; 3,121,124; 3,482,952; 3,726,942; 3,940,450 and 4,039,590, the disclosures of which are incorporated herein.
It is known that methyl tertiary butyl ether (MTBE) can be manufactured by an acid catalyzed addition reaction of methanol with isobutene. Catalysts which have become widely used for the acid catalyzed addition reaction are sulphonated organic resins, such as disclosed in U.S. Pat. No. 2,480,940, and especially the sulphonated polystyrene resins crosslinked with divinylbenzene, as disclosed in U.S. Pat. No. 3,922,822, which can be of a gelatinous nature or can possess a sponge structure with macropores, in order to increase the surface area and thus to increase the rate of reaction. British Pat. No. 957,000, Example 8, and U.S. Pat. No. 3,482,952 disclose the method of increasing the surface area.
Since the reaction between methanol and isobutene proceeds very selectively, it is in general not pure isobutene, but an isobutene-containing mixture of hydrocarbons which is employed. In particular, the crack C.sub.4 hydrocarbon cut freed from butadiene--that is to say the so-called raffinate I--is employed for the reaction. However, other isobutene-containing mixtures of C.sub.4 -hydrocarbons can also be used such as disclosed in U.S. Pat. Nos. 3,121,124 and 4,039,590.
When raffinate I is employed for the manufacture of MTBE, the unconverted residue of the hydrocarbon mixture is termed raffinate II. When raffinate II is employed as the starting material for the manufacture of further products, such as, for example, maleic anhydride or methyl ethyl ketone, or for obtaining 1-butene or in polymerization reactions, the standards of quality demanded of raffinate II are high. In particular in the case of its use for obtaining 1-butene from raffinate II by distillation, the isobutene content of raffinate II must be substantially less than 1% and preferably even less than 0.25 percent by weight, since the isobutene cannot be separated off from 1-butene by distillation and therefore remains in its entirety in the 1-butene. However, a maximum content of isobutene of less than 0.25 percent by weight in raffinate II means that when the isobutene is removed by reaction an isobutene conversion of at least 99.75% must be achieved.
When isobutene is reacted with methanol to give MTBE, the product is not pure MTBE, but only MTBE/methanol azeotrope and methanol. Solutions to the problems of separating the methanol from the MTBE have already been proposed, for example by an extractive distillation with dimethylsulphoxide, as disclosed in Japanese published application No. 73-00509, now Japanese Pat. Sho No. 48-509 or by a water wash, as disclosed in British Pat. No. 1,369,889, and U.S. Pat. No. 3,726,942. In U.S. Pat. No. 3,940,450, the separation of methanol and MTBE is carried out in two stages using pentane as an auxiliary material. All of these processes are relatively involved, since the auxiliary materials employed have to be removed again virtually completely from the MTBE and the methanol. The isobutene conversions achievable are unsatisfactory. In the process according to U.S. Pat. No. 3,726,942, only 70% of the isobutene is converted to MTBE, while according to U.S. Pat. Nos. 3,940,450, a MTBE yield of 80% is achieved.
U.S. Pat. No. 3,979,461 and 4,071,567 describe processes for the manufacture of MTBE by reacting isobutene containing mixtures of C.sub.4 -hydrocarbons with methanol in two reaction zones. However, even with these two processes, adequately high isobutene conversions are not achievable in a conrolled manner, where the lowest isobutene content is given as 0.27% in Example 6 of U.S. Pat. No. 3,979,461. In order to achieve high isobutene conversions in the manufacture of methanol-free MTBE it is necessary to carry out the reaction in one of the two reaction stages using high reactor temperatures or a molar excess of isobutene over methanol. As a result of this, however, oligomerization products of isobutene, dimerization and trimerization, also form in addition to MTBE and, in an undesired manner, a considerable isomerization of the C.sub.4 -olefins in the mixture of hydrocarbons takes place. In any subsequent distillation which may be carried out to isolate 1-butene from the C.sub.4 mixture, the isomerization of 1-butene to 2-butene has the effect of a product loss.
U.S. Pat. No. 4,219,678 does indeed describe a one-stage process for the manufacture of MTBE, in which the working up of the reaction mixture is carried out without the use of auxiliary materials and which does not have the abovementioned disadvantages, such as the formation of oligomerization products of isobutene and isomerization of the unconverted C.sub.4 -olefins, but the maximum isobutene conversion which can be achieved is only about 98.6% so that the residual C.sub.4 mixture (raffinate II) contains more than 1% of isobutene.